This invention relates generally to a heat radiating coating for a refractory core and a method of manufacture thereof, and particularly relates to such a coating suitable, for example, for the rotating anode of an X-ray tube, high power tubes and the like, and capable of withstanding the high vacuum and high temperatures encountered in such environments.
For many applications such as high power tubes and X-ray tubes, it is imperative to have a material capable of radiating heat with high efficiency. For example, in an X-ray tube of the type having a rotating anode, the electron beam bombards a track on the anode which in turn radiates the X-rays. However, most of the incoming energy is transformed into heat. Hence the efficiency of the reradiation of heat from the anode determines how often the tube can be pulsed or how much energy can be applied thereto as a function of time.
For example, for modern X-ray techniques rapid and repetitive exposures are required to obtain the desired results. This is particularly true of angiography or cine-radiography. Similar considerations apply for high power tubes.
In the past various materials capable of withstanding high temperatures have been used, for example, for the anode or the anode coating of X-ray tubes. Generally, such materials as tungsten and rhenium or composites thereof have been proposed. These materials not only are capable of withstanding high temperatures, but are also good X-ray emitters due to their high atomic numbers.
For example, the U.S. Pat. No. 3,649,355 to Hennig describes a process for coating a refractory core for the purpose of providing a rotating X-ray anode. It is proposed that tungsten or tungsten alloys are deposited from a gaseous phase and such vapor deposition to produce tungsten coatings is generally known. The refractory core may, for example, consist of graphite or other suitable materials.
The U.S. Pat. No. 2,863,083 to Schram also discloses anodes for X-ray tubes. Specifically, it is proposed to make the anode either entirely of rhenium or to provide it with a coating of rhenium. One of the processes recommended is the chemical vapor deposition of rhenium by decomposing a halogenide of rhenium such as rhenium pentachloride at a temperature between 500 and 1500.degree.C (centigrade) in vacuum. However, the emissivity of such a coating disclosed by Schram is no more than about 0.3. It is also proposed by Schram that the non-focal surface be blackened by a known process not further specified, apparently in an effort to increase the heat radiation properties of the coating.
The anode may consist of a refractory core, for example molybdenum or graphite. In some cases an intermediate layer is provided between the outer rhenium coating and the refractory core and the intermediate layer may, for example, consist of tungsten or the like.
Reference is also made to the prior U.S. Pat. to Kaplan et al. No. 3,819,971. This patent deals with a composite anode for X-ray tubes where the coating is retained to the core by means of scoring the surface of the core or similar means. This patent also discloses a process for the chemical vapor deposition of rhenium, for example, from rhenium hexafluoride or rhenium pentachloride. The rhenium hexafluoride reaction is carried out at 800.degree.C and for the rhenium pentachloride at about 1000.degree.C. However, none of these prior coatings have a really high emissivity, where the maximum possible emissivity is 1.0 as in a perfect black body radiator. In other words, the actual emissivity is less than one third of the theoretical emissivity.
It is accordingly an object of the present invention to provide a coating consisting of rhenium for a refractory core which has superior heat radiating properties, that is an emissivity on the order of 0.9.
Another object of the present invention is to provide such a coating which consists of radiation reflecting needle-like elements which due to their steep angles will reflect radiation out of the coating to provide a high degree of radiation and a correspondingly high emissivity.
A further object of the present invention is to provide a process of depositing such a heat radiating coating of rhenium by the vapor deposition of the rhenium in the form of a halogenide of rhenium.